The present invention relates to processes and devices that utilize capillary forces to separate fluids. Several of the inventive embodiments are limited to microcomponent or microchannel devices that utilize capillary forces.
Compact systems for capturing and/or separating fluids are desirable in a variety of applications. For example, hydrogen-powered vehicles could utilize fuel cells that recycle water. As another example, efficient and lightweight systems for recovery and reuse of water in spacecraft has long been recognized as a requirement for human space exploration. The present invention provides methods and apparatus for efficient fluid capture and separation.
The invention provides methods and apparatus for separating fluids and/or heat exchange. One process separates fluids by passing a mixture of at least two fluids, comprising a first fluid and a second fluid, into a device having at least one channel. The channel has an open area and a wicking region. The first fluid is either a liquid (such as a droplet or liquid particle) that is sorbed by the wicking region, or a gas that, under separation conditions, forms a liquid in the wicking region. The first liquid travels through the wicking region to a liquid flow channel and then exits the device through a liquid exit channel. The second fluid is a gas that passes through the gas flow channel to a gas exit, and exits the device through the gas exit.
The invention also provides a process of contacting fluids in which at least two fluids are passed into a device having at least one channel. The channel has an open area and a wicking region and an interface between the wicking region and the open area. During operation, at least one fluid flows through the wicking region, and at least one other fluid flows through the open area. At the interface between the wicking region and the open area, one fluid contacts at least one other immiscible fluid, and there is mass transfer occuring through the interface between the at least one fluid flowing through the wicking region, and the at least one other fluid flowing through the open area.
The invention further provides a method of condensing a liquid in which a gas passes into a device having at least one channel. The channel has an open area and a wicking region and is in thermal contact with at least one microchannel heat exchanger; and a heat exchange fluid is passed through the microchannel heat exchanger(s). During operation heat is removed from the gas stream causing some part to condense to form a liquid. Said formed liquid is sorbed into the wicking region, travels through the wicking region to a liquid flow channel and then exits the device through a liquid exit channel.
The invention further provides an apparatus having at least one channel comprising an open area and a wick. The wick in the channel is connected to an exit wick, and the open area is connected to a gas exit. This apparatus is useful for many of the processes described herein.
The invention also provides a liquid condenser comprising at least one channel; wherein the channel comprises a gas flow channel and a wick. The channel is in thermal contact with at least one microchannel heat exchanger. Both the apparatus and condenser are particularly well suited for use in a chemical reactor.
The presence of wicks and optional pore throats and capture structures are common to multiple embodiments of the invention. A wick is a material that will preferentially retain a wetting fluid by capillary forces and through which there are multiple continuous channels through which liquids may travel by capillary flow. The channels can be regularly or irregularly shaped. Liquid will migrate through a dry wick, while liquid in a liquid-containing wick can be transported by applying a pressure differential, such as suction, to a part or parts of the wick. The capillary pore size in the wick can be selected based on the contact angle of the liquid and the intended pressure gradient in the device, and the surface tension of the liquid. Preferably, the pressure at which gas will intrude into the wick should be greater than the pressure differential across the wick during operation-this will exclude gas from the wick.
The liquid preferentially resides in the wick due to surface forces, i.e. wettability, and is held there by interfacial tension. The liquid prefers the wick to the gas channel and as long as there is capacity in the wick, liquid is removed from the gas stream and does not leave in the gas stream.
The wick can be made of different materials depending on the liquid that is intended to be transported through the wick. The wick could be a uniform material, a mixture of materials, a composite material, or a gradient material. For example, the wick could be graded by pore size or wettability to help drain liquid in a desired direction. Examples of wick materials suitable for use in the invention include: sintered metals, metal screens, metal foams, polymer fibers including cellulosic fibers, or other wetting, porous materials. The capillary pore sizes in the wick materials are preferably in the range of 10 nm to 1 mm, more preferably 100 nm to 0.1 mm, where these sizes are the largest pore diameters in the cross-section of a wick observed by scanning electron microscopy (SEM). In a preferred embodiment, the wick is, or includes, a microchannel structure. Liquid in the microchannels migrates by capillary flow. The microchannels can be of any length, preferably the microchannels have a depth of 1 to 1000 micrometers (xcexcm), more preferably 10 to 500 xcexcm. Preferably the microchannels have a width of 1 to 1000 xcexcm, more preferably 10 to 100 xcexcm. In a preferred embodiment, the microchannels are microgrooves, that is, having a constant or decreasing width from the top to the bottom of the groove. In another embodiment, the microchannels form the mouth to a larger diameter pore for liquid transport.
The wick is preferably not permitted to dry out during operation since this could result in gas escaping through the wick. One approach for avoiding dryout is to add a flow restrictor in capillary contact with the wick structure, such as a porous structure with a smaller pore size than the wick structure and limiting the magnitude of the suction pressure such that the non-wetting phase(s) cannot displace the wetting phase from the flow restrictor. This type of restrictor is also known as a pore throat. In preferred embodiments, a pore throat is provided between the wick and the liquid flow channel and/or at the liquid outlet. In some embodiments, the wick can have a small pore diameter such that is serves to transport fluids from the gas channel and also prevents gas intrusion, thus serving the dual purpose of a wick and a pore throat.
A pore throat has a bubble point that is greater than the maximum pressure difference across the pore throat during operation. This precludes intrusion of gas into the pore throat due to capillary forces (surface tension, wettability, and contact angle dependent). The pore throat should seal the liquid exit, so there should be a seal around the pore throat or the pore throat should cover the exit in order to prevent gas from bypassing the pore throat. The pore throat is preferably very thin to maximize liquid flow through the pore throat at a give pressure drop across the pore throat. Preferably, the pore throat has a pore size that is less than half that of the wick and a thickness of 50% or less than the wick""s thickness; more preferably the pore throat has a pore size that is 20% or less that of the wick. Preferably, the pore throat is in capillary contact with the wicking material to prevent gas from being trapped between the wick and the pore throat and blocking the exit.
Flooding can result from exceeding the flow capacity of the device for wetting phase through the wick; the flow capacity is determined by the pore structure of the wick, the cross-sectional area for flow, or the pressure drop in the wick in the direction of flow.
A capture structure can be inserted (at least partly) within the gas flow channel, and in liquid contact with the wick. The capture structure assists in removing (capturing) a liquid from the gas stream. One example of a capture structure are cones that protrude from the wick; liquid can condense on the cones and migrate into the wickxe2x80x94an example of this capture structure is shown in U.S. Pat. No. 3,289,752, incorporated herein by reference. Other capture structures include inverted cones, a liquid-nonwetting porous structure having a pore size gradient with pore sizes getting larger toward the wick, a liquid-wetting porous structure having a pore size gradient with pore sizes getting smaller toward the wick and fibers such as found in commercial demisters or filter media. Mechanisms for capturing dispersed liquid particles include impingement (due to flow around obstructions), Brownian capture (long residence time in high surface area structure), gravity, centrifugal forces (high curvature in flow), or incorporating fields, such as electrical or sonic fields, to induce aerosol particle motion relative to the flow field.
Nonwetting surfaces can be disposed on the gas flow channel walls. These nonwetting surfaces can help prevent formation of a liquid film on the surface and, in combination with a wick or a wick and capture structure the liquid present in a fluid mixture can be siphoned away from the condensing surface by capillary flow, thereby avoiding problems associated with dropwise condensation, such as cold spots or re-entrainment.
The invention, in various aspects and embodiments can provide numerous advantages including: rapid mass transport, high rates of heat transfer, low cost, durability, and highly efficient liquid separations in a compact space.
Devices and processes of the present invention are capable of integrating high efficiency, high power density heat exchange. Heat exchange can facilitate phase changes within the separation device, such as condensation and evaporation. One example is partial condensation of a gas stream to recover condensable components, such as water from the cathode waste gas stream from a fuel cell. Another optional feature is reduced or non-wettability of the wall adjacent to a heat exchange surface to preclude formation of a liquid film. The heat transfer coefficient would increase substantially by avoiding the resistance of a liquid film.
The embodiments show preferred embodiments in which there are multiple gas flow channels operating in parallel. This configuration allows high throughput and provides a large surface area to volume ratio for high efficiency. In some preferred embodiments, layers are stacked to have between 2 and 600 separate gas flow channels, more preferably between 4 and 40 gas flow channels. As an alternative to the parallel arrangement, the channels could be connected in series to create a longer flow path.
Another advantageous feature of some preferred embodiments of the invention is that the gas flow channels and/or liquid flow channels are essentially planar in the fluid separation regions. This configuration enables highly rapid and uniform rates of mass and heat transport. In some preferred embodiments, the gas flow channels and/or liquid flow channels have dimensions of width and length that are at least 10 times larger than the dimension of height (which is perpendicular to net gas flow).
The subject matter of the present invention is distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may further be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.
A xe2x80x9ccapture structurexe2x80x9d is a structure disposed (at least partly) within a gas flow channel that assists movement of a liquid into the wick.
A xe2x80x9ccellxe2x80x9d refers to a separate component, or an area within an integrated device, in which at least one unit operation is performed. In preferred embodiments, the cell has a width less than about 20 cm, length less than about 20 cm, and height less than about 3 cm.
xe2x80x9cDevice volumexe2x80x9d refers to the entire volume of the device, including channels, headers, and shims.
xe2x80x9cEntrainmentxe2x80x9d refers to transport of liquid into the gas exit.
xe2x80x9cFlow microchannelxe2x80x9d refers to a microchannel through which a fluid flows during normal operation of an apparatus.
A xe2x80x9claminated devicexe2x80x9d is a device having at least two nonidentical layers, wherein these at least two nonidentical layers can perform a unit operation, such as heat transfer, condensation, etc., and where each of the two nonidentical layers are capable having a fluid flow through the layer. In the present invention, a laminated device is not a bundle of fibers in a fluid medium.
A xe2x80x9cliquidxe2x80x9d is a substance that is in the liquid phase within the wick under the relevant operating conditions.
xe2x80x9cMicrochannelxe2x80x9d refers to a channel having at least one dimension of 5 mm or less. The length of a microchannel is defined as the furthest direction a fluid could flow, during normal operation, before hitting a wall. The width and depth are perpendicular to length, and to each other, and, in the illustrated embodiments, width is measured in the plane of a shim or layer.
xe2x80x9cMicrocomponentxe2x80x9d is a component that, during operation, is part of a unit process operation and has a dimension that is 1 mm or less.
xe2x80x9cMicrocomponent cellxe2x80x9d is a cell within a device wherein the cell contains microcomponents.
xe2x80x9cPore throatxe2x80x9d refers to a porous structure having a maximum pore dimension such that a non-wetting fluid is restricted from displacing a wetting fluid contained with the pore throat under normal operating conditions.
xe2x80x9cResidence timexe2x80x9d refers to the time that a fluid occupies a given working volume.
xe2x80x9cUnit process operationxe2x80x9d refers to an operation in which the chemical or physical properties of a fluid stream are modified. Unit process operations (also called unit operations) may include modifications in a fluid stream""s temperature, pressure or composition.
A xe2x80x9cwicking regionxe2x80x9d is the volume occupied by a wick, or, a wicking surface such as a grooved microchannel surface.
xe2x80x9cWorking volumexe2x80x9d refers to the total channel volume of the device, and excludes the headers and solid shim and end plate materials.